Epidemiologic data suggest that vitamin D may play a role in the pathogenesis of prostate cancer. Moreover, in vitro and animal studies have shown that vitamin D inhibits the growth of prostate cancer cells. This study was designed to examine the potential clinical utility of vitamin D in treating men with early stages of prostate cancer. Men were enrolled into this study if, after primary treatment of their prostate cancer (surgical prostatectomy or radiation therapy), they demonstrated a rising serum level of the prostate tumor marker, Prostatic Specific Antigen (PSA). They must have had stage A or B disease and not have evidence of metastasis, and therefore have minimal recurrent disease. The primary endpoint of the study is to show a slowing of the rate of rise of PSA levels as an index of slowed prostate cancer growth while on vitamin D therapy (calcitriol, Rocaltrol-Roche). Eight otherwise healthy men have thus far been enrolled in this study. One subject was excluded prior to the initiation of therapy due to the identification of potential metastatic disease. Seven subjects have been initiated on calcitriol therapy. Two subjects were withdrawn from therapy due to the development of kidney stones. One subject was withdrawn from the study because his physician initiated androgen ablation therapy, although there was no objective evidence of cancer progression in this individual. All 7 subjects have shown a beneficial response to the therapy, with a slowing of the PSA rate of rise compared to the rate of rise before therapy was initiated. The slowing was significant in every case as well as the group as a whole. The baseline PSA doubling time in each subject differed, the response to calcitriol was at least a doubling of the doubling time. In some patients the doubling time increased five-fold. Although recruitment has closed, study is ongoing in 2 men because of the beneficial response to therapy as indicated by slowed rate of PSA rise.